This invention relates generally to the production of pitches and more particularly to a process for continuous production of a pitch by a liquid-phase heating treatment of a heavy hydrocarbon oil, in which the liquid-phase heating treatment is carried out successively in a plurality of reactors connected in series, at least one of the reactors having a recirculation line.
The procedure of subjecting a heavy hydrocarbon oil derived from petroleum or coal (hereinafter referred to simply as "heavy oil ") to a high temperature for a specific time thereby to cause thermal cracking and polymerization and condensation of the hydrocarbons, to distill out gases and light oil fractions, and, at the same time, to produce as a residue a pitch, which, in general, is a black solid at room temperature, is known.
The character of this pitch differs with conditions such as the kind of the starting material oil and the process temperature and time. It is known that pitches which have been subjected to heat treatment at high temperature over a long period have a high content of components insoluble in solvents and become pitches of low content of volatile matter and, at the same time, high softening point. It is also a matter of common knowledge that pitches produced in this manner are used in a wide range of applications, a few examples thereof being as a binder for the manufacture of formed carbon products such as carbon electrodes and formed coke, as an oil adsorbent, as a starting material for various carbon products, and as a bituminous additive for various materials such as plastics and concrete.
Accordingly, various processes for producing these useful pitches with heavy oil as starting material have been and are being proposed. Among these known processes, there are two principal types. One of these types is a vapor-phase process in which heavy oil is subjected to a vapor-phase contact process for a short time together with a heating medium such as steam at a high temperature of the order of 800.degree. C to 2,000.degree. C (as disclosed, for example, in Japanese Patent Publication No. 27844/1975 and Patent Laid Open No. 21003/1975). The other type is a liquid-phase process in which heavy oil is heat treated over a long time at a relatively low temperature of the order of 300.degree. C to 500.degree. C (as disclosed, for example, in Japanese Patent Publication 43103/1974 and Patent Publication 22422/1972). Also known are combinations of these two types (as disclosed, for example, in Japanese Patent Publication 2417/1971).
Of these known processes, the vapor-phase process can be said to be intrinsically suited for continuous production of pitch, but it entails difficult problems in reduction to an industrial-scale operation such as severity of conditions of contact between the starting material heavy oil and the heating medium, conditions relating to the nozzle construction and arrangement, and conditions of heating. On the other hand, the liquid-phase process has the intrinsic advantage of being carried out under mild reaction conditions. Furthermore, since the liquid-phase process produces pitches of characteristics differing from those resulting from the vapor-phase process, there is a necessity of developing this process independently of the vapor-phase process. Accordingly, various proposals as cited above are being made.
However, almost all of the processes proposed heretofore for producing pitches by the liquid-phase process are of the batch type, and there have been few attempts to develop a continuous process. The reason for this evidently is that it is necessary to hold the heavy oil at the reaction temperature for a relatively long time in the case of a liquid-phase process, and in order to meet this requirement, a reaction process of the batch vessel type is inherently better suited. In addition, as the heating and transformation of the starting material oil into a heavier oil progresses, coking occurs simultaneously, and it becomes difficult to avoid adhesion of carbonaceous materials to the reaction vessel wall parts and clogging of the outlet piping.
For example, in the delayed coking process, a heavy oil of the same character as that in the production of pitch is used as a starting material, which is treated for a long time at a temperature in a similar range, thereby to produce coke. As is apparent from this, also, the occurrence of coking is an inevitable problem in the production of pitch. In the delayed coking process, incidently, as a method of recovering the formed coke, the reactor is opened, and complicated steps such as boring and cutting of the solidified coke are resorted to.
This coking and adhesion of formed carbonaceous materials to the wall and other parts of the reactor, although not desirable because of deviations in the character of the product pitch, do not constitute a fatal obstacle to the operation in a batch process. In a continuous process, however, they are fatal, rendering the operation practically impossible.
With the aim of overcoming the difficulties caused by this partial coking of heavy oil, various methods have been proposed. One method (e.g., as disclosed in the specification of Japanese Patent Laid Open No. 59102/1973), although it relates to a batch process, comprises dividing the reaction into two stages, carrying out the first-stage reaction with respect to heavy oil with which solid carbon particles have been admixed, causing carbonaceous substance formed to adhere to the carbon particles, subjecting these solid carbon components to solid-liquid separation thereby to separate an oily substance, which is then subjected to the second-stage heating and transformation into a heavier oil or pitch.
However, a method of this character involving solid-liquid separation requires a complicated operation as described above and, in view of the restriction of the operational conditions by the properties of the coke formed at the same time, cannot be said to be always suitable for continuous and, at the same time, economical production of pitch having a broad range of properties.